Why won't my new battery charge?

Not sure where you bought the battery but if it has been sitting on the sellers shelf for awhile it may have discharged. Li-ion batteries will discharge if sitting to long and have a protective circuit in them that will turn the battery off if over discharged which usually happens around 2.9v to 2.2v.

The battery is not dead though and can be woken back up. You can find battery chargers that have a wake-up feature or boost feature to reactivate and charge batteries that have fallen asleep. A regular charger is not able to wake the battery up(not enough voltage) so there for it can not charge it. The boost feature applies a small charge current to first activate the protection circuit and then begins supplying a normal charge. I know Cadex makes these types of chargers.

You could also try and boost it yourself. There are plenty of videos on youtube that show you how you can do this. I only suggest doing this if you have a multimeter and know how to use it to check batteries voltage. These internal batteries are a lot harder to do this to than removable batteries like Samsung uses. You basically take a usb charging cable that hooks into your pc or wall and cut off the end with the micro usb and expose its wires(while unplugged). Next strip the black and red wires ends. I took the ends of the wires and attached them to an extra set of multimeter probes. I put them on the ends that plug into the multimeter. Now you plug in the usb cable and take the red wire and put it to your batteries positive and take the black wire and put it to your batteries negative. After 30 seconds to a minute, it should boost your batteries voltage back to a chargeable state. I have done this with numerous batteries that read 0v with a tester and I got it up to 3.9v or what ever range I needed within a minute or so. If you do this make sure the battery doesn't get to warm. If this procedure or the boost charger does not work than you were sold a bad battery. Or you may need to check you usb charge port on the phone to make sure its clean or that it is not faulty.

Good luck and hope this information helps.